It is known that many integrated circuits use protections against short-circuits to control the maximum current that flows through the power transistors. In some applications it is important that the power transistor be able to supply a small current also when high voltages are applied to its terminals.
In single power-supply audio applications, during power-on transients the power transistor must charge the decoupling capacitor, which is connected between the loudspeaker and the output of the power stage, while the entire power supply voltage is applied to the terminals of the power transistor.
In such a situation, it is important to ensure that the power transistor is capable of supplying the current required to charge said capacitor, so that the amplifier can power-on correctly.
FIG. 1 illustrates a conventional protection circuit, in which the reference numeral 1 designates the power transistor and the reference numeral 2 designates a so-called sense resistor, which is suitable to sense the output current Io and is arranged in series to the power transistor 1, a current mirror Q1, Q2 with corresponding current sources Io and Iref which are series-connected, and finally a circuit branch which is connected between the ground and the bases of the transistors Q1 and Q2 and is constituted by a resistor 3 which is series-connected to a Zener diode 4.
At a point that is intermediate between the transistor Q2 and the current source Iref, a pin is provided for acquiring the signal for indicating intervention or lack thereof on the power transistor 1 in order to limit the maximum voltage across it.
The resistor 3 senses the voltage drop on the power transistor 1. When he voltage across the transistor exceeds the value ##EQU1##
the protection circuit disconnects the power transistor 1 and no current can be supplied anymore: this limits the maximum supply voltage.
In the above relation, the terms A2 and A1 are, respectively, the areas of the transistors Q2 and Q1, while Vds_Max is the maximum voltage between the drain and the source of the power transistor 1 and V.sub.z is the voltage across the Zener diode 4.